In the injection molding of plastic material it is common to utilize a hot runner system in which the machine nozzle or injection unit is disengaged from the hot runner as part of the cycle. During the injection phase of the cycle, the molten plastic resin in the hot runner is subjected to high injection pressure. Before the injection unit can be disengaged from the hot runner, however, this high injection pressure must be dissipated so that when the injection unit is separated from the hot runner, molten plastic is not expelled under pressure from the hot runner supply orifice. This expulsion of plastic from the hot runner on separation of the injection unit is normally termed "drool".
Prevention of drool is usually accomplished by "suckback". The injection unit is capable of injecting molten plastic at high pressure by virtue of a hydraulic piston pushing the plastic through the nozzle of the injection unit. Suckback occurs when the hydraulic piston is retracted instead of pushing the plastic and thereby causes a negative pressure in the machine nozzle or suckback. The retraction of the piston occurs prior to separation of the injection unit from the hot runner system and therefore the connected hot runner system is also depressurized or sucked back into the injection unit.
Theoretically perfect application of suckback leaves the hot runner system completely depressurized so that upon separation of the injection unit from the hot runner system no molten plastic will be expelled from either the hot runner supply orifice or from the mold gates, if open. However, this requires perfect application of suckback which may not be achieved. Also, resin compressibility for some resin can be as high as 20%. Only a very short period of time during the molten cycle is available for suckback and although the majority of depressurizing may be achieved, sufficient residual pressure and/or resin compressibility may remain to cause resin from the hot runner supply orifice upon separation of the injection unit therefrom. This has presented a significant problem in this art.
Various measures have been proposed to eliminate drool, as for example, a ball check valve or shut off valve. However, these are not entirely satisfactory. They represent an undesirable flow restriction in the supply channel of the hot runner. It causes a pressure drop and provides places for heat sensitive resins to hang-up and/or degrade. A shut-off valve requires time to operate in a depressurized condition, and in frequently operated cycles, valves like this tend to leak.
Another alternative is the use of an anti-drool bushing in the sprue bars of stack molds. These machines generally require the frequent separation of the machine nozzle from the hot runner supply orifice. The anti-drool bushing uses a sliding bushing in the sprue bar plus suckback. The bushing is pushed outward by the residual internal hot runner pressure in the manifold and sprue bar. In so doing, an increased volume of space is provided for the decompression resin, thus reducing drool. Upon re-engagement of the machine nozzle, the bushing is pushed back into the sprue bar. The disadvantage of the anti-drool bushing is that it does not actually close the orifice and therefore can still permit drool, albeit in reduced amounts.
It is therefore a principal object of the present invention to provide an improved injection molding apparatus which reduces drool when the injection unit is separated from the hot runner system, and which generally eliminates the drool entirely.
It is a still further object of the present invention to provide an improved apparatus as aforesaid which is simple and convenient to operate even in a high speed cycle and which is not accompanied by adverse side effects, as increased pressure drop or undesirable flow path restrictions.